Improve DuckDB::ScalarFunction C API coverage

[suketa]

Overview

DuckDB::ScalarFunction is missing several features from the DuckDB C API. This issue tracks the full list.

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Missing API methods

DuckDB::ScalarFunction

• set_varargs(logical_type) — wrap duckdb_scalar_function_set_varargs. Allows accepting a variable number of arguments of a given type.
• set_special_handling — wrap duckdb_scalar_function_set_special_handling. Marks the function to receive NULL inputs.
• set_bind(&block) — wrap duckdb_scalar_function_set_bind. Enables dynamic return type resolution based on input types at bind time.

DuckDB::FunctionInfo

• get_bind_data — wrap duckdb_scalar_function_get_bind_data. Allows the execute callback to retrieve data stored at bind time.
• set_error (scalar-specific) — wrap duckdb_scalar_function_set_error. Currently only the generic duckdb_function_set_error is wrapped.

New class: DuckDB::ScalarFunctionBindInfo

The bind callback receives a duckdb_bind_info. Scalar-function-specific methods not yet wrapped:

• set_bind_data(data, destroy_callback) — duckdb_scalar_function_set_bind_data
• set_bind_data_copy(copy_callback) — duckdb_scalar_function_set_bind_data_copy
• set_error(message) — duckdb_scalar_function_bind_set_error
• get_extra_info — duckdb_scalar_function_bind_get_extra_info
• argument_count — duckdb_scalar_function_bind_get_argument_count
• get_argument(index) — duckdb_scalar_function_bind_get_argument
• get_client_context — duckdb_scalar_function_get_client_context

New class: DuckDB::ScalarFunctionSet

Enables registering multiple type overloads under the same function name:

• DuckDB::ScalarFunctionSet.new(name) — duckdb_create_scalar_function_set
• #add(scalar_function) — duckdb_add_scalar_function_to_set
• Connection#register_scalar_function_set(set) — duckdb_register_scalar_function_set

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Missing type support in input/output handling

Currently handled: BOOLEAN, TINYINT, UTINYINT, SMALLINT, USMALLINT, INTEGER, UINTEGER, BIGINT, UBIGINT, FLOAT, DOUBLE, VARCHAR, BLOB, TIMESTAMP, DATE, TIME

Missing scalar types (straightforward)

• HUGEINT, UHUGEINT — use duckdb_hugeint / duckdb_uhugeint struct → Ruby Integer
• TIMESTAMP_S, TIMESTAMP_MS, TIMESTAMP_NS, TIMESTAMP_TZ — same pattern as TIMESTAMP
• TIME_TZ, TIME_NS — same pattern as TIME
• INTERVAL — map to/from DuckDB::Interval
• UUID — stored as hugeint internally, expose as String
• DECIMAL — needs width/scale metadata from logical type

Missing complex types (harder, lower priority)

• LIST, STRUCT, MAP, ARRAY, UNION, ENUM — require recursive child vector handling

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Note: the same type gaps also exist in DuckDB::DataChunk#set_value used by the table function adapter, so fixing these would benefit both APIs.